Hay Belly – No Topline, What Does This Mean and How to ManageJanuary 2018

Story by: Don Kapper

The mainstay of all diets for horses, over four months of age, is the forage they consume. Understanding how the equine digestive tract functions optimally and how forage quality and quantity can help maintain the health of the horse, reduce the incidence of colic, and keep horses growing, reproducing and performing up to their genetic potential is what every horse owner, manager, trainer should know. Horses have an absolute requirement for forage, but have no requirement for cereal grains (oat, barley, corn, etc.) The only reason to feed a ‘horse feed’ or a ‘balancer’ to horses is to make up the difference between what nutrients are in their forage (hay and pasture), and what they need to meet their daily nutrient needs, while maintaining their ‘optimal’ body condition.

Hay Belly?

A term ‘hay belly’, or distended abdomen, is often incorrectly used to describe an overweight horse, when the rest of the body is normal. FACT: Fat does not accumulate extensively on the horse’s lower abdomen (belly). Horses do accumulate fat in specific areas: neck (crest), behind the shoulders, over the ribs and around the tail-head. See Dr. Henneke’s Body Condition Scoring system (BCS) that categorizes the horses condition and is based on the amount of fat stored in these different areas. Several things can cause a distended abdomen, but the most common cause is feeding over-mature hay. This is because as hay matures, the percentage of lignin increases to make the plant stem stronger, to hold its flower or seed heads. Lignin is that portion of the fiber that is not fermentable or digestible, therefore, it slows the rate of passage through the digestive system. All of the ingesta must enter the cecum at the top, ferment, break down, then rise to the top again to exit into the large intestine. The cecum is called the ‘blind gut’, because the entrance and exit are located side by side, at the top. The fibers that do not break down with fermentation, do not rise to the top and can cause a distention of the abdomen. Therefore, we do not recommend feeding any hay that is a Grade 5, (or a RFV below 74), due to the increased incidence of impaction colic.

Poor Topline!

Your horse’s topline and overall muscle development play a critical role in how well your horse can perform their daily tasks. Horses with ‘atrophied’ topline muscles, will appear: a) sunken in or concave along their back, and/or b) loin, and/or c) concave around their hip bones and hindquarters, and therefore, will not be able to perform up to their genetic potential.

For year’s horses have been supplemented with added calories (fat, fiber and cereal grains) in an effort to correct poor top-lines. This practice has provided only limited success. The reason for this is simple: Calories provide the fuel for horses to perform. If more calories are consumed than needed, excess fat is deposited behind the shoulders, over the ribs, on their crest (neck) and around the tail head. Not until a Body Condition Score >7 is achieved, does extra fat get deposited over the topline muscles. Unfortunately, this does not improve performance.

Exercise has also been attributed to developing a horse’s topline. While exercise can condition and define the existing muscles, it has very little to do with “building” the topline area, since the increase in muscle mass occurs with ‘resistance’ training. After adequate topline muscles have been built, the ability of the horse to perform may then be determined by how well he is conditioned through exercise.

Building the Topline

Horses require specific nutritional “building blocks” to develop the muscles all over their body. These building blocks are called ‘amino acids’ and there are ‘10 Essential’ ones that must be supplied every day in their diet. Four of them needed in the largest amounts are: lysine, methionine, tryptophan and threonine. All 21 amino acids are linked together in their body to form and develop muscles. Since 95% of muscles are Amino Acids, one ‘limiting’ amino acid can affect the availability of all of them and effect the development of muscles all over their body. The easiest area to recognize an amino acid deficiency is their ‘toplines’. Diets containing adequate levels of all the essential amino acids can quickly improve the imperfect topline. Then, through exercise and conditioning, the topline can be strengthened and the desired level of performance achieved.

Table 1: An Example of How the First Limiting Amino Works

First Limiting Amino Acid clip-art

Because the percentages of the ‘Essential’ amino acids decrease with the maturity of the hay, they can be added individually or are found in specific ingredients, such as soybean meal or milk products, i.e. whey. These primary sources are commonly used throughout the world for humans and horses. The addition of even small amounts of essential amino acids can pay huge dividends in building your horse’s muscles all over their body, including their topline.


The construction and function of the horse’s digestive system provides limited ability to use poor quality forages, so everyone must understand how to determine quality and identify different ‘types’ of forages in their respective area. Because the ‘maturity’ of the plant when harvested affects the digestibility (fermentability) of the fiber as well as the availability and amount of protein, calories, major and trace minerals, we must be able to visibly identify hays from: being too mature, to the highest quality. Feeding the most immature forages to your horses who need the highest amount of nutrients per day is vital, most cost effective and is the best management practice. That would be the: upper level performance horses, nursing mares, sucklings and weanlings. For the ‘easy-keepers’ (over-weight) horses, selecting a more mature hay would be ideal because it will contain a lower number of calories per pound.

The Hay Market Task Force of the American Forage and Grassland Council has published a quality grading standard based on the maturity of the forage when harvested. The FORAGE GRADE is determined by the percentage of different fibers when analyzed. Table 2 provides the different percentages of fiber at different maturity levels of the plant. All forage labs report the Acid Detergent Fiber (ADF), that measures crude fiber (cellulose plus insoluble lignin) plus soluble lignin, and Neutral Detergent Fiber (NDF), that adds the ADF and Hemicellulose together. As plants mature, their percentage of fibers increase and all the other nutrient percentages decrease at the same rate. Forage labs can also provide the Relative Feed Value (RFV), which is one number for you to use to determine quality, while other labs provide a Forage Grade. The following chart shows how much less the horse can eat per day due to the slower rate of passage through their digestive system as the fiber percentages increase and their fermentation rate slows. The percentage listed on the right side of the following chart is based on the ‘potential’ Dry Matter Intake (DMI) as a percent of the mature horses Body Weight.

Table 2: The Smaller Number of the FORAGE GRADE, the more Palatable and Digestible it is and the more horses can eat per day.

The table below lists:
a) the different Forage Grades (Prime thru 5)
b) ranges of ADF %
c) ranges of NDF %
d) their resulting Relative Feed Value number (RFV)
e) potential Hay Intake per day, as a % of their Body Weight (DMI/day)

Forage Grade   If ADF % is: If NDF % is: Then RFV is: Hay DMI/day is:
Prime Under 30  Under 40 Over 151 >3.0%
1 31-35  41-46 150-125 2.9-2.7%
2 36-40 47-53 124-103 2.6-2.4%
3 41-42  54-60 102-  87  2.3-2.1%
4 43-45 61-65 86-  75 2.0-1.8%
5 (Reject) Over 46 Over 66   Under 74 <1.8%

This table is from the veterinary textbook, Equine Internal Medicine, 2nd Edition, ‘Applied Nutrition’ Chapter, p.1544. Donald R Kapper, PAS, author and Stephen M Reed, DVM, editor

Horses need the optimum balance between all the ‘essential’ amino acids

Dr. Larry Chase, Dept. of Animal Science, Cornell University, has published the different values of the ‘essential’ amino acids found in the different ‘Types’ of forages (found in pasture and hay). We have utilized this new information to help us determine what levels of amino acids need to be added into horse feeds and balancers to provide the optimum balance required every day, based on the ‘TYPE’ of hay being fed and the amount of food recommended to be fed/day.

Table 3: is from Cornell’s Equi-Analytical Laboratory. It contains the ‘Nutrient Ranges’ found in over 90,000 grass hay samples, from 2000 to 2016. It lists the highest and the lowest nutrients found, then provides the ‘average’ of all grass hay analysis. The disturbing over-view is the highest quality grass hay was a Grade 2, while the lowest was a Grade 5. ‘Good’ quality grass hay is a Grade 2 (based on fermentability), while the average over this 16 year period, was a Grade 3, and there were no ‘Grade 1’ or ‘Prime’ hays analyzed. This information states horse people need to learn what ‘good’ hay really is and how to identify it visually and by analysis.

Table 3: Nutrients, DM Basis: Lowest Average Highest
(Grade 5)
(Grade 3)
(Grade 2)
Acid Detergent Fiber (ADF) 43.66% 38.90% 34.12%
Neutral Detergent Fiber (NDF) 69.54% 62.58% 55.62%
Starch 0.36% 1.76% 3.15%
ESC (Simple Sugars, Ethanol Sol. Carbs) 5.58% 7.09% 9.60%
WSC (Water Soluble Carbs) 6.89% 11.50% 16.10%
Crude Protein 6.96% 10.86% 14.76%
Lysine, based on grass forage 0.23% 0.36% 0.50%
= 0.34% of Crude Protein, DM
Digestible Energy kcal/lb 811 909 1,008
Digestible Energy kcal/kg 368 413 458
Calcium, (Ca) 0.27% 0.49% 0.72%
Phosphorus, (P) 0.15% 0.24% 0.33%

This chart can also help identify potential nutritional deficiencies, when ‘over-mature’ hay is fed to horses. You can see how the Crude Protein and its corresponding percentage of Lysine decrease rapidly at Grade 4. You can be assured all 10 of the essential amino acids will decrease accordingly. Look also as the number of calories that drop as the percentage of fibers increase. The major minerals also fall into this rapid decline when the hay becomes over-mature. Because Phosphorus is a key metabolite involved in almost every metabolic pathway, you can see why the horse’s health can also be negatively affected when over mature hay, i.e. Grade 5, is fed.

The maturity of the plants when harvested is the most important consideration when producing top-quality hay, i.e. Grade 2 or better. If hay is cut after it is mature, there is nothing you can change to improve the quality (digestibility of the fibers) in that forage. The time allotted for forages to go from Prime to Grade 5 in the cool seasons of spring and autumn is 4 to 5 weeks. In the hot summer months in the northern states, or the entire growing season in the southern states, complete maturation takes less that 3 weeks. This faster maturing is directly related to the ambient temperature and it is not very forgiving.


Every feed manufacturer is responsible to include:

1) a ‘purpose statement’ on each feed, to explain what physiological status of horses this formula is to be fed;
2) what the ‘minimum’ amount is to feed per day to meet their nutrient needs, and
3) what ‘type of forage’ their feed is formulated to complement (grass or legume). The amount to feed per day will vary based on the horse’s size, along with their age (i.e. growth rate), reproductive status and performance level. If less ‘horse feed’ if fed per day than what is recommended on their Feeding Directions, they could be feeding a nutrient deficient diet.


Since forage should be 70% to 90% of the mature horse’s diet, it is very important owners realize how important it is to change the horses forage slowly. Remember, when feeding hay, you are providing food to feed the microflora in their ‘fermentation vat’, or hind gut, and the microflora feed the horse. It takes 21 days to make a complete microbial change, so go slow when changing hays. If you are changing from grass pasture to grass hay, or from grass hay to alfalfa hay, take 10 days to 2 weeks to make this change, by mixing them together during this time. IF you are purchasing new grass hay to replace last year’s grass hay, take at least 7 days to make the transition. This is because you must consider the health of the microbes, and it takes different microbes to break down the different ‘types’ of forages. Because it is the horse’s enzymes that will break down ‘horse feed’ (protein and cereal grains) so the horse can utilize them, it is more important for all managers to change their hay more slowly than changing their grain mixtures or balancers.

If the hay just purchased is too mature, be prepared to see the horse’s abdomen begin to enlarge, and if you have been relying on the crude protein in the hay to maintain muscle mass, be prepared to see their muscles begin to atrophy, or shrink, due to the lower percentage of amino acids. This more mature hay is also lower in calories, so you may have to increase the amount of ‘horse feed’ fed per day to help maintain desired body weight. The easiest place to witness the decrease in protein, i.e. amino acids, is on their topline. Learn how to provide a Topline Evaluation Score (TES) once a month and record. The easiest places to witness a decrease in body weight is rib cover, tail-head fat deposit, amount of fat behind the shoulder and the size of their crest. Learn how to Body Condition Score (BCS) once a month and record. Then adjust your feed and feeding program accordingly.

Table 4 is a good chart for you to use when evaluating the best ‘quality’ of hay for your horses. Spending a few dollars to determine the Forage Grade or RFV before you buy, can potentially save you money by maintaining the health of your horse’s digestive system, reduce the chance of colic and improve your horse’s immune response.

Table 4: Hay Management Chart: Using the ‘Forage Grade’, or the ‘Relative Feed Value’, when Selecting Hays to Match the Physiological Status of your Horses

Calorie Needs:  Forage Grade or Best RFV to Feed
1) Highest Need 1 or Over 125
High Performance, Lactating, Sucklings, Weanlings
2) Medium Need 2 or 103 to 124
Eng. & West. Pleasure, Low Level Dressage, Yearlings
3) Lowest Need 3 and 4 or 75 to 102
Mature Idle, Lay-ups, ‘Special Needs’, over-weight, etc.
4) Avoid feeding 5 or Below 74
Because there is a high incidence of ‘impaction colic’ due to the amount of ‘over-mature and indigestible’ fiber

Don KapperDon Kapper is a highly experienced equine nutritionist and a member of the Cargill Equine Enterprise Team. Don graduated from Ohio State University and achieved his credentials as a Professional Animal Scientist from the American Registry of Professional Animal Scientists in 1996 and has been a sought-after speaker for equine meetings in both the U.S. and Canada. He was a member of the “Performance Electrolyte Research” team at the University of Guelph and wrote the chapter on “Applied Nutrition” for the authoritative veterinary textbook: “Equine Internal Medicine”, 2nd edition. Don also co-developed the “Equine Nutrition” course for the Equine Science Certificate program for Equine Guelph and has been a popular guest speaker in several Equine Guelph online courses, including the Equine Growth and Development, Exercise Physiology and Advanced Equine Nutrition.